Because of shortcomings of metallic plates, bioabsorbable, polymeric plates have been developed for fracture fixation in bone surgery. E.g. elongated, bioabsorbable, six-hole plates were developed by Eitenmüller et al. for orthopaedic animal studies (European Congress on Biomaterials, Abstracts, Instituto Rizzoli, Bologna, 1986, p. 94). However, because of inadequate strength, some of the fracture fixation plates were broken in animal experiments.
U.S. Pat. No. 5,569,250 describes a biocompatible osteosynthesis plate operable for being enhanced in a substantially secured relation to a plurality of adjacent bone portions. The osteosynthesis plate is in a first configuration at a first thermochemical state and is operable to be converted to a second thermochemical state so that it may be deformed prior to fixation.
The first thermochemical state is typically room temperature (operation room conditions) and the second thermochemical state is typically an elevated temperature above Tg of the polymer material (e.g. for polylactides between 50-60° C.). Accordingly, the plates of U.S. Pat. No. 5,569,250 must be changed from the first thermochemical state to the second thermochemical state, to be shaped (deformed) and thereafter they must be changed again back to the first thermochemical state prior to fixation. Because the thermal conductivity of polymeric materials is poor, the conversion of material to a second temperature is a slow process. Therefore, the clinical use of plates of U.S. Pat. No. 5,569,250 is tedious, slow and complex especially if the surgeon must shape the plate several times to make it fit exactly to the form of the bone to be fixed.
K. Bessho et al., J. Oral. Maxillofac. Surg. 55 (1997) 941-945, describe bioabsorbable poly-L-lactide miniplate and screw system for osteosynthesis in oral and maxillofacial surgery. Also these plates must be heated by immersion in a hot sterilized physiologic salt solution or by the application of hot air until they become plastic and can only then be fitted to the surface of the bone.
EP 0 449 867 B1 describes a plate for fixation of a bone fracture, osteotomy, arthrodesis etc. said plate being intended to be fixed on bone at least with one fixation device, like screw, rod, clamp or corresponding, wherein the plate comprises at least two essentially superimposed plates, so as to provide a multilayer plate construction, so that the individual plates of said multilayer plate construction are flexible so as to provide a change of form of said multilayer plate construction to substantially assume the shape of the bone surface in the operation conditions by means of an external force such as by hand and/or by bending instrument directed to said multilayer plate construction, whereby each individual plate assumes the position of its own with respect to other individual plates by differential motion along the coinciding surfaces.
Although the said multilayer plate fits even the curved bone surface without heating of individual plates, the clinical use of multilayer plate is tedious, because the single plates easily slip in relation to each other before fixation. Additionally the thickness of multilayer plate system easily becomes too thick for cranio maxillofacial applications, causing cosmetic disturbance and increased risks for foreign body reaction.
EP 0 987 033 A1 corresponding to U.S. Pat. No. 6,632,503 describes a biodegradable and bioabsorbable implant material wherein its shape after deformation within ordinary temperature range can be fixed and maintained so that its shape can be easily adjusted at the site of operation, and it has substantially no anisotropy in view of strength. Particularly, it provides an implant material which can effect deformation such as bending or twisting within ordinary temperature range and has a shape-keeping ability to fix and maintain the shape after deformation as such, wherein molecular chains, domains of molecular chain assembly or crystals of the polymer are oriented along a large number of reference axes having random axial directions.
In manufacturing the implant material, an injection molded or extrusion molded billet is first pressed at low temperature to a bottom-closed forming mold having smaller dimensions that the billet, to prepare a forged molding block (first forging step), and the forged molding block, as such or after cutting into an appropriate size, is then forged a second time in a mechanical direction that is different from the direction of pressing in the first forging step. According the document, in the second time forging the crystals of the polymer, which have been oriented in parallel along many reference axes in the first forging step, are subjected to rearrangement in the mechanical direction so that the many reference axes direct toward various directions randomly. As a result, the crystals of the polymer are oriented along a large number of reference axes having different axial directions, or clusters having these reference axes having different orientation are assembled in a large number. The small orientation units of molecular chains, domains of molecular chain assembly or crystals of EP 0 987 033 A1, form a non-continuous, random (non-directed) reinforcement into the material, analogous with the random short-fiber or whisker reinforcement. However, it should be advantageous to have in the oriented plate material a multiaxial continuous orientation, while long, continuous orientation units give usually better mechanical property combination for the material than short ones.
Random axial directions of small orientation units of molecular chains, domains of molecular chain assembly or crystals could make the material structure unfavorable to resist tear (plate cutting) loads between holes with fixation screws, which tear loads originate from the tendency of bone fragments to draw apart in relation to each others (typically in the plane of the flat surface and long axis of the plate).
Even if the implant material of EP 0 987 033 A1 can be bended or twisted within ordinary temperature range, it is manufactured with a complex non-continuous process including first melt molding and thereafter secondly and thirdly, or even more non-continuous forging (solid state molding) steps.
U.S. Pat. No. 6,221,075 and U.S. Pat. No. 6,692,497 describe bioabsorbable osteosynthesis plate and its surgical use. The plate is made of a material that is oriented uni- and/or biaxially and is substantially rigid and deformable at temperatures below the glass transition temperature of the material.
Uniaxially oriented plate material has good mechanical strength in the tensile mode, in the direction of orientation but it is strongly anisotropic so that longitudinal splitting of the material is a risk, when multiaxial mechanical forces are stressing the plate. Also biaxial orientation yields materials with anisotropic mechanical properties, because orientation structure is not uniform in different directions.
US Pat. Appl. published as US 2007/270852 A describes a bioabsorbable surgical osteosynthesis plate that is substantially rigid and deformable at temperatures below the glass transition temperature (Tg) of the material, that plate having multiaxial spiral orientation.
The multiaxial spiral orientation is received by turning the ends of an elongated, solid preform (billet) to opposite directions so that the preform twists itself along its long axis. When compressing the spiral oriented preform to a plate-like billet at a temperature above the glass transition temperature, Tg, of the material but below its melting temperature, Tm (if any), a multiaxial spiral oriented plate preform is obtained. Even if the spiral oriented, multiaxially oriented plate has not some of the limitations of uni- or biaxially oriented plates, it is impossible to manufacture broad plates, like mesh-plates with this method, due to the delamination of material layers inside of the plate structure caused by shear forces developed in the compression molding of the material to form a plate out of the cylindrical preform. Delamination of the material can be seen as fracture surfaces inside of the plate, parallel with the broad surfaces of the plate.
Therefore, there is a need of bioabsorbable (bioresorbable or biodegradable) oriented material or plate, which:                can be deformed, yet is dimensionally stable at temperatures below Tg of the material        has more isotropic mechanical properties than uni- and biaxially oriented materials have        does not possess the tendency to internal delamination        can be made also in the form of broad mesh-plates.        
A need also exists for a bioabsorbable (bioresorbable or biodegradable) broad osteosynthesis material and plate, which is strong and tough, does not produce a substantial inflammatory response, which has isotropic tear load carrying capacity in different directions of the plate surface and has a good resistance against longitudinal splitting and against internal delamination and which material or plate can be deformed, yet is dimensionally stable at temperatures below Tg of the material from which the device is made, to facilitate shaping.
A need also exists for such a bioabsorbable (bioresorbable or biodegradable) osteosynthesis material and plate, which is broad, strong, tough, does not produce a substantial inflammatory response, which has an isotropic resistance against splitting and against internal delamination which material or plate can be deformed, yet is dimensionally stable, in operation room conditions, to facilitate the shaping of the plate.
A need also exists for such a material and plate, which is thin (the thickness typically from 0.1 mm up to 6 mm), long and broad, so that it can be used as or formed to a mesh plate with the length (l) and the width (w) of several centimeters (e.g. l=10 cm, w=10 cm).
A need also exists for such a material and plate, which has the length and the width of several centimeters, so that the material can be cut advantageously with mechanical and/or thermomechanical methods to smaller osteosynthesis plates.
A need also exists for such a bioabsorbable (bioresorbable or biodegradable) osteosynthesis material and plate, which is strong, tough, does not produce a substantial inflammatory response, which has an isotropic resistance against splitting in all directions of the surface plane and against internal delamination and which material and plate can be deformed, yet is dimensionally stable in operation room conditions (in the first thermochemical state) to allow its fixation on bone without distortion of the configuration of the bone fragments to be fixed, and which shaped plate is also dimensionally stable in tissue conditions (in the second thermochemical state), when fixed on bone surface to facilitate non-problematic healing of bone fracture.
A need also exists for such a bioabsorbable osteosynthesis plate preform, which has a good resistance against splitting (tear) in different directions of the surface plane and which plate preform can be processed economically (rapidly and effectively) to smaller plates e.g. with mechanical cutting or laser cutting, giving smaller plates with isotropic mechanical properties.